[PATCH v3 bpf-next] selftests/bpf: Fix arena_spin_lock selftest failure

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

For systems having CONFIG_NR_CPUS set to > 1024 in kernel config
the selftest fails as arena_spin_lock_irqsave() returns EOPNOTSUPP.
(eg - incase of powerpc default value for CONFIG_NR_CPUS is 8192)

The selftest is skipped incase bpf program returns EOPNOTSUPP,
with a descriptive message logged.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
---

Changes since v2:
* Separated arena_spin_lock selftest fix patch from the arena
  patchset as it has to go via bpf-next tree.
* For EOPNOTSUPP set test_skip to 3, to differentiate it from
  scenarios when run conditions are not met as suggested by Hari.
* Tweaked message displayed on SKIP to remove display of online
  cpus.

v2:https://lore.kernel.org/all/20250829165135.1273071-1-skb99@linux.ibm.com/

Changes since v1:
Addressed comments from Alexei:
* Removed skel->rodata->nr_cpus = get_nprocs() and its usage to get
  currently online cpus(as it needs to be updated from userspace).

v1:https://lore.kernel.org/all/20250805062747.3479221-1-skb99@linux.ibm.com/

---
 .../selftests/bpf/prog_tests/arena_spin_lock.c      | 13 +++++++++++++
 tools/testing/selftests/bpf/progs/arena_spin_lock.c |  5 ++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c b/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
index 0223fce4db2b..693fd86fbde6 100644
--- a/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
+++ b/tools/testing/selftests/bpf/prog_tests/arena_spin_lock.c
@@ -40,8 +40,13 @@ static void *spin_lock_thread(void *arg)
 
 	err = bpf_prog_test_run_opts(prog_fd, &topts);
 	ASSERT_OK(err, "test_run err");
+
+	if (topts.retval == -EOPNOTSUPP)
+		goto end;
+
 	ASSERT_EQ((int)topts.retval, 0, "test_run retval");
 
+end:
 	pthread_exit(arg);
 }
 
@@ -63,6 +68,7 @@ static void test_arena_spin_lock_size(int size)
 	skel = arena_spin_lock__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "arena_spin_lock__open_and_load"))
 		return;
+
 	if (skel->data->test_skip == 2) {
 		test__skip();
 		goto end;
@@ -86,6 +92,13 @@ static void test_arena_spin_lock_size(int size)
 			goto end_barrier;
 	}
 
+	if (skel->data->test_skip == 3) {
+		printf("%s:SKIP: CONFIG_NR_CPUS exceed the maximum supported by arena spinlock\n",
+		       __func__);
+		test__skip();
+		goto end_barrier;
+	}
+
 	ASSERT_EQ(skel->bss->counter, repeat * nthreads, "check counter value");
 
 end_barrier:
diff --git a/tools/testing/selftests/bpf/progs/arena_spin_lock.c b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
index c4500c37f85e..086b57a426cf 100644
--- a/tools/testing/selftests/bpf/progs/arena_spin_lock.c
+++ b/tools/testing/selftests/bpf/progs/arena_spin_lock.c
@@ -37,8 +37,11 @@ int prog(void *ctx)
 #if defined(ENABLE_ATOMICS_TESTS) && defined(__BPF_FEATURE_ADDR_SPACE_CAST)
 	unsigned long flags;
 
-	if ((ret = arena_spin_lock_irqsave(&lock, flags)))
+	if ((ret = arena_spin_lock_irqsave(&lock, flags))) {
+		if (ret == -EOPNOTSUPP)
+			test_skip = 3;
 		return ret;
+	}
 	if (counter != limit)
 		counter++;
 	bpf_repeat(cs_count);
-- 
2.43.5